1. Field of Invention
This invention relates to fire proofing of wood substrates and, in particular, to a method of treating wood substrates to impart flame resistance thereto.
2. Brief Statement of the Prior Art
Flameproofing treatments of wood have generally used intumescent compositions such as tri-ammonium phosphate, polyhydric alcohols and mitrogenous materials. Upon exposure to the flame, the ammonium phosphate decomposes, absorbing heat to decrease the temperature of the coated item. Frequently, the gases released from intumescent coatings are toxic.
Halogenated polymers have also been used as flameproofing agents and are applied by pressure impregnation, a costly method. Bromine and chlorine are most commonly used in epoxy polymers, polyesters and vinyl polymers. Additionally, inorganic materials such as zinc borate and antimony oxides are added to decrease the flammability of the coatings. As with intumescent coatings, these halogenated polymer coatings also release highly toxic gases. Flame resistance of wood substrates has also been achieved to some degree by pressure impregnation of wood with silicates, phosphates and borates. The flame resistance achieved with these treatments is not entirely permanent as the materials are lost from the substrate upon weathering, and this loss is particularly acute with topically applied coatings which lack the limited degree of permanency of impregnated coatings. Additionally, toxicity of the decomposition products can be a problem with these flame retardants also.
Alumina trihydrate is an ideal material for achieving flame retardants since it releases water vapor when heated above about 280 degrees F. This water vapor release absorbs heat from the advancing flame and protects the substrate. Heretofore, however, there has been no coating developed which traps the water vapor released from alumina trihydrate to form a foam which contains water vapor and pyrolysis gases beneath a refractory and thermal insulating barrier over the wood substrate.